The present invention is directed to semiconductor lasers. More particularly, the present invention is directed to a method for improving a single frequency semiconductor laser comprising driving said laser with an optimized current pulse.
Fiber optical communication systems operating at 1.5 .mu.m over long distances with non-dispersion shifted fibers require single frequency lasers with narrow linewidths and strong side mode suppression. If a laser is biased at, or slightly above, threshold and pulse code modulated at high data rates (over several hundred megabits/sec) with the usual type of current pulse, severe oscillations of the injected carriers are produced. This is the well-known relaxation oscillation phenomenon which is predicted by the semiconductor laser rate equations.
The relaxation oscillations of the injected carrier density produce two undesirable effects in lasers which are designed for single frequency transmission: (1) the oscillations produce increased optical power in the side modes, which leads to mode partition noise in the communication system; (2) the oscillation of the carrier density produces oscillation in the center frequency of the lasing line, which causes increased pulse dispersion. It has recently been shown that in a single frequency cleaved couple cavity (C.sup.3) laser operating at 1 Gb/s over a distance of 120 km a 5.6 dB powder penalty resulted from the increased width of the laser line. See for example, R. A. Linke, Electronics Letters, 20: 472 (1984). Similar effects have been observed in distributed feedback (DFB) lasers. See for example, I. Miro et al., Electronics Letters, 20: 261 (1984).